Fix for #34911, Compositor with multiple views autozooms to node when creating a link. In order to allow node trees have different offsets in different editor instances, the view_center is now stored primarily in the bNodeTreePath struct for each tree in a node space. The view_center in bNodeTree is only used as an initial setting when opening a node group or switching node trees.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56115 954f8c5b-7b00-dc11-b283-0030488c597c